Ethylene homopolymers and copolymers of ethylene with higher α-olefins such as 1-butene, 1-pentene, 1-hexene or 1-octene can be prepared, for example, by polymerization using supported titanium compounds, known as Ziegler-Natta catalysts, or else using supported chromium compounds, known as Phillips catalysts. When the homopolymers and copolymers of ethylene are used, for example, for blown film extrusion, it is important that the polymers have a good balance between mechanical properties and processability.
Ethylene homopolymers and copolymers of ethylene with higher α-olefins such as 1-butene, 1-pentene, 1-hexene or 1-octene can be prepared, for example, by polymerization using supported titanium compounds, known as Ziegler-Natta catalysts, or else using supported chromium compounds, known as Phillips catalysts. When the homopolymers and copolymers of ethylene are used, for example, for blown film extrusion, it is important that the polymers have a good balance between mechanical properties and processability.
It is known that supported chromium catalysts are very suitable for producing ethylene copolymers having good mechanical properties. The properties of the polymers obtained in the polymerization are dependent on the way in which the chromium catalyst used has been prepared, in particular on the type of support material, e.g. its chemical structure, composition, surface area or pore volume, the type of chromium compound used, the presence of further compounds, e.g. titanium compounds, aluminum alkyls or carbon monoxide, the order in which the various components are applied or the manner of calcination and activation. It is a combination of the starting materials used together with the procedure for application to a support which then gives the desired chromium catalyst for the preparation of polymers having the property profile required for the specific application.
The supported chromium catalysts are often titanized, i.e. they comprise not only the chromium compound but also variable proportions of a titanium compound by means of which the molar mass distribution and the HLMI (high load melt index), for example, can be influenced. The application of the titanium compound to the support is usually carried out during the preparation of the hydrogel, giving an SiO2—TiO2 cogel. In this, the titanium dioxide is uniformly distributed throughout the support material. A disadvantage is that only a fraction of the total titanium oxide is available for polymerization at the pore surface of the catalyst. For this reason, numerous embodiments of titanized chromium catalysts in which the titanium compound is applied in a targeted manner to the pore surface, usually in a step separate from the doping of the chromium compound, have been developed.
Thus, for example, EP-A-882740 describes a process for preparing a supported chromium catalyst, in which the support material has a specific surface area of from 450 to 600 m2/g and the chromium component is applied to the support first and the titanium compound is applied subsequently, with the titanization being carried out at temperatures of at least 300° C.
EP-A-882741 teaches that polyethylenes having favourable ultimate tensile strengths are obtained when using a supported chromium catalyst whose support material has a specific surface area of at least 400 m2/g and has been dehydrated before use and in the preparation of which the chromium component is applied to the support first and the titanium compound is applied subsequently.
The application of a mixture of a chromium compound and a titanium compound in an aprotic solvent to a support under aprotic conditions is described in JP 54141893 and JP 57049605.
However, the preparation and handling of organometallic compounds under aprotic conditions is complicated and costly, since the solvents have to be dried before use. In addition, only few chromium compounds are soluble in aprotic media. An increase in the solubility of chromium compounds in aprotic solvents can often only be achieved by means of a complicated synthesis.